Cavity resonator



March 1, 1949;. r M, B 2,463,472

CAVI-TY RESONATOR Filed Marchle, 1945 j Z0 I FI F- 3 DH um HENRY M- BAcHPatented Mar. 1, 1949 ()FFICE CAVITY RESGNATOR Henry M. Bach, Lawrence,N. Y., assignor to Premier Crystal Laboratories, Incorporated,

New York, N. Y.

Application March 16, 1945, Serial No. 583,066

1 3 Claims.

ihis invention relates to cavity resonators, and

more particularly, to mechanically modulated cavity resonators.

A main object of this invention is to provide a cavity resonator adaptedto be employed in ultra high frequency radi equipment as a tanl; circuitelement for generating a frequency-modulated ultra high frequency wavein a transmitter or as a resonator for tuning a receiver to an ultrahigh frequency wave.

A further object of the invention is to provide a resonator device foran ultra high frequency receiver which is adapted to providesubstantially panoramic tuning for the receiver whereby all signals in adesired band of frequencies in the microwave region may besimultaneously detected.

Further objects and advantages of the inven tion Will appear from thefollowing description and claims, and from the accompanying drawings,wherein:

Figure 1 is a diagrammatic view of a cavity resonator of the squareprism type provided with a frequency-modulating means in accordance withthis invention Figure 2 is a diagrammatic view of a cavity resonator ofthe toroidal type provided with irequency-modulating means in accordancewith this invention.

Figure 3 is a diagrammatic view illustrating a modification of a cavityresonator frequencymodulating structure in accordance with thisinvention.

Cavity resonators as ordinarily used in ultra high frequency wavetechnique are employed as tank circuit elements for generating highfrequency waves in. combination with appropriate electron tubes, astuned filter elements in amplifiers, as selective circuits in receivers,in electromagnetic horns, and in other devices requiring sharp resonancecharacteristics at a predetermined frequency or frequencies.

In the conventional cavity resonator the dimensions of the cavity arefixed so that the resonance effect occurs at a substantially distinctfrequency, or provision may be made for adjus ing one or more dimensionsor by adjusting a tuning element within the cavity to vary the resonancefrequency. After adjustment is made the resonator is still characterizedby a substantially distinct single-frequency response, as a generalrule.

In certain applications of ultra high frequency waves it is desirable tofrequency-modulate the cavity resonator at a relatively rapid andconstantly maintained rate, such as in certain types of signallingbetween aircraft, or between aircraft and, ground stations; fordetermining range distances by measurement of phase differences betweentransmitted and reflected Waves; for calibrating range oscillators; andfor a large number of other radio navigational applications. In certaininstances it may be desirable to amplitude-modulate thefrequency-modulated wave, such as in certain types of televisiontransmissions.

In the reception of frequency-modulated ultra high frequency waves asabove generated, it is necessary to frequency modulate the selectivecavity resonator tuning elements of the receiver at the same modulationrate as the transmitter for proper reception. However, under certaincircumstances it i desirable to provide for panoramic reception of alldistinct waves (not frequency-modulated) in a given band in the ultra.high frequency region, Where provision is made for separating andindicating the respective waves as they exist over the band. Withoutreference to the specific details of the equipment for separating andindicating the respective waves present in the band, it will be clearthat a receiver having frequency-modulated cavity resonator tuningelements which are frequency-modulated over the desired band will atcertain periodically related instants be in resonance with each of therespective waves (not frequency-modulated) which exist over the band.

It is therefore a prime purpose of this invention to provide afrequency-modulated cavity resonator adapted for use in equipmentsimilar to that above mentioned.

Referring to the drawings, Figure 1 discloses a cavity resonator of thesquare prism type wherein l designates the body portion thereof whichmay be of copper or other highly conductive material of substantialthickness. At one end wall 4 of the cavity an opening 2 is provided forthe insertion of a coupling loop 3. Adjacent the other end wall of thecavity the body portion l is formed with a rim portion 5 0isubstantially reduced thickness which is in abutment with a squarepiezo-electric crystal plate 6, secured to rim by soldering the edgeportions of its inner electrode l to the adjacent inner wall surfaces ofthe cavity. Electrode l is of the plated type and is sufliciently thickto adequately rigidly hold and support crystal plate 6 against rim 5,the soldered joints providing a substantially rigid connection betweensaid rim and the crystal.

The outer electrode 8 of the crystal is also of the plated type.Electrode 8 is connected to the grid of an appropriate oscillator tube9, the cathode of said tube being connected to body portion 1 of thecavity resonator, which is conductively connected to electrode 7 by thesoldered joint at rim 5. An appropriate plate tuning circuit is providedfor oscillator tube 9 for oscillating crystal 6 in an extensional mode,as indicated by the dotted lines in Figure 1, whereby the thickness ofthe crystal as well as the length thereof alternates in accordance withthe period of oscillation of the crystal. Rim 5 is dimensioned to be ofan appropriate length and thickness so as to vibrate in phase withcrystal 6 so that there is no relative motion between rim 5 and crystal6 at the soldered joint connecting them. The resultant motion of crystal6 with respect to body portion l of the cavity resonator is such thatelectrode 1 oscillates laterally and periodically changes its distancefrom end wall 4 at a rate equal to the frequency of oscillation ofcrystal 6. The periodic change in the dimension between wall 4- andelectrode l frequency-modulates the cavity resonator at a frequency ofmodulation substantially equal to the crystal oscillation frequency. Thedegree of modulation depends upon the specific dimensions of theresonator.

In the embodiment of Figure 2 a metal toroidal cavity resonator II] isemployed having a first reentrant wall ll through which the couplingloop I2 is introduced and a second reentrant surface element l3 oppositewall I! comprising the inner plated electrode of a circularpiezoelectric crystal plate l4. Crystal plate M is mounted on anappropriately formed annular rim structure l5 provided on the toroid,and surface element I3 is peripherally soldered to the inner surface ofannular rim l'5. An outer plated electrode I6 is provided for crystall4. said outer electrode being connected to the grid of an oscillatortube H. The cathode of tube I! is connected to toroid Ill and anappropriate plate tuning circuit is provided for tube [1 for oscillatingcrystal M in an extensional mode. As in the embodiment of Figure 1, rimI5 is dimensioned to vibrate freely in phase with crystal it so that norelative motion occurs between the crystal and said rim at theperipheral soldered joint. The resultant oscillatory motion of surfaceelement I3 with respect to wall H frequency-modulates the cavityresonator substantially at the oscillation frequency of the crystal, thedegree of mod.-

ulation being somewhat greater than in the em bodiment of Figure 1 for agiven amplitude of crystal oscillation due to the closer normal spacingbetween the wall H and surface element #3 of the toroid cavityresonator.

In Figure 3 a modification is disclosed wherein the crystal I8 issupported within the cavity resonator. The cavity resonator is formedwith an opening l9 in a wall thereof. The crystal is of a size and shapesuch as to slightly overlap the edges of opening 19 when positioned oversaid opening at the inner surface of said wall. A first electrode 20 isprovided on crystal it which provides a continuous conductive surfacefor that portion of the crystal exposed to the inside of the cavityresonator, said electrode being rigidly secured to the apertured cavitywall by an appropriate peripheral soldered joint. A second electrode Mis provided on that portion of the crystal i8 exposed to the exterior,electrode 24 being insulated from electrode 28 by an appropriate gapbetween the electrodes.

The rim portion of the resonator need not be designed to oscillate inmechanical resonance with the crystal, inasmuch as the crystal may beappropriately oriented and shaped so as to be substantially stationaryat its peripheral edge portions while expanding and contracting inthickness to thus produce the frequency-modulating effect in the cavityresonator.

Other suitable electrical means for oscillating the crystal than thespecific circuit herein described may be employed within thecontemplation of this invention. Thus, a grid-plate crystal oscillatorcircuit or any other crystal oscillator circuit known in the art may beemployed, or any other known electrical means for driving the crystal.

It is also contemplated that the vibrating surface element may be drivenby mechanical vibrators such as tuning fork devices or bymagne-tostriction oscillator devices.

It is further contemplated that the vibratory element may be mounted atother points within the cavity than at a wall portion thereof, since thefrequency-modulation effect may be achieved wherever the vibratoryelement can be coupled in any manner to the electromagnetic fieldexisting within the cavity.

While certain specific embodiments of mechanically modulated cavityresonators have been disclosed in the foregoing description, it will beunderstood that various modifications within the spirit of the inventionmay occur to those skilled in the art.

What is claimed is:

1. A cavity resonator having as a wall element a piezo-electric crystalunit having plated electrodes on the opposite faces thereof, oneelectrode facing inwardly of the cavity resonator and the otherelectrode facing outwardly, the inwardly facing electrode beingconnected to the cavity resonator body at its peripheral edges, wherebyoscillation of the piezo-electric crystal unit periodically varies theresonance frequency of the cavity resonator.

2. A cavity resonator having as a wall element a piezo-electric crystalunit having plated electrodes on the opposite faces thereof. oneelectrode facing inwardly of the cavity resonator'and the otherelectrode facing outwardly, the inwardly facing electrode beingconnected to the cavity resonator body at its peripheral edges and saidresonator body being substantially reduced in wall thickness immediatelyadjacent to its connection to the peripheral edges of said inwardlyfacing electrode, whereby oscillation of the piezo-electric unitperiodically varies the resonance frequency of the cavity resonator.

3. A cavity resonator having as a wall element thereof a piezo-electriccrystal unit comprising a piezo-electric crystal plate oriented andshaped so as to be substantially stationary at its periph eral edgeportions while expanding and contracting in thickness duringoscillation, and having plated electrodes on the opposite faces thereof,one electrode facing inwardly of the cavity resonator and the otherelectrode facing outwardly,

2,463,472 5 6 the inwardly facing electrode being connected at UNITEDSTATES PATENTS its peripheral edges to the cavity resonator body,whereby expansion and contraction in thickness 2 82? Name Date of thecrystal plate during oscillation periodically 2 174 701 gfi gig variesthe resonance frequency of the cavity 5 2233263 Lmder 1941 resonator. i

2,241,976 Blewett May 13, 1941 HENRY BACH- 2,306,282 Samuel Dec. 22,1942 2,323,201 Carter June 29, 1943 REFERENCES CITED 0 2,374,810 FremlinMay 1, 1945 The following references are of record in the 1 2,405,277Thompson Aug. 6, 1946 file Of this patent: 2,408,425 Jenks Oct. 1, 1946

